This invention relates to a method of deep drawing hard metal material such as magnesium.
Heretofore, there have been distributed extensively various kinds of plastic products and, as a result, a large quantity of plastic refuse and waste have been generated or dumped. Plastic waste has been incinerated together with combustible waste, which results in a serious problem of generation of atmospheric pollution and other environmental problems. Moreover, plastic products, even if they are withdrawn for the purpose of recycling, have disadvantages in that the cost associated with their recycling is high, whereby recycling thereof into new products is not profitable. Therefore, a rate of withdrawal of plastic products has been extremely low.
There has been advocated the use of magnesium material as substitutes or alternates for the plastic material, and casting of magnesium material has been proposed. However, cast products of magnesium are limited in their shape. Further, if deep drawing is carried out by press working, it is likely that products are cracked and, therefore, it has been difficult to provide successful press working.
An attempt has been made to overcome the difficulties as described above by using a press working die which is heated by a suitable electric heater or the like to about 250xc2x0 C. However, this has a disadvantage in that dies must be heated every time a press working operation is to be carried out, which results in a serious drawback with regard to working efficiency. Further, a magnesium plate is increasingly hardened as it is repeatedly subjected to press working; and thus it becomes difficult to deep draw a magnesium plate and moreover, there is a serious problem in that cracks and seams are generated on surfaces of the formed products.
An object of the present invention is to provide a new deep drawing method for magnesium plates.
Another object of the invention is to provide a new deep drawing method for magnesium plates, which are for structural purposes and have good recycling characteristics, environmental compatibility and energy saving properties.
According to an aspect of the present invention, there is provided a deep drawing method comprising using a die having a male mold with a convex forming surface and a female mold with a concave forming surface, comprising the steps of:
placing a vertically movably mounting table, which supports thereon a magnesium plate, between the male mold and the female mold,
placing a plurality of thin resin films on the magnesium plate,
keeping a central portion of the magnesium plate contacted with the convex forming surface of the male mold and moving the concave forming surface of the female mold relative to the convex forming surface of the male mold, thereby achieving a deep drawing of the magnesium plate.
In the deep drawing method according to the present invention, a layer of the thin resin films consists of at least two vinyl films each having a thickness of 0.02 mm. Also, it is preferred that the magnesium plate has a thickness of about 1.5 mm.
In the deep drawing method of the present invention, a pressure is applied to the magnesium plate by a base plate which is removably fitted on an upper surface of the concave forming surface of the female mold, while a vertical movement of the mounting table is adjusted by an extension device such as a hydraulic cylinder.
Further, in the deep drawing method of the present invention, the female mold has four female elements having an L-shaped cross section and fitted on a lower surface of the base plate. Each of the four female elements has a concave forming surface at its upper side of each inner corner surface for forming (i.e., for secondary molding) purposes, and an outwardly dilated guide surface, for molding purposes, continuously extending from the lower side of the concave forming surface.
Further, in the present invention, the female mold has four female elements having an L-shape cross section, and each inner corner surface has a concave forming surface. Each of the female elements is radially movably fitted to a lower surface of the base plate with a resilient element being mounted therein so that each of the female elements can be moved radially within a predetermined range. Further, supporting members each being positioned between adjacent female elements are fixed to the lower surface of the base plate to restrict a recovery position of the female elements, and the female elements are recovered to their original positions by a spring force of the resilient element that biases the female elements which have been radially pushed outwardly by the male element.
A shrinkage restriction plate can be fitted to either the male mold or female mold.
In the present invention described above, pre-pressing or molding (i.e., primary forming) is carried out by the outwardly dilated guide concave surface at the lower side of the female mold before further fabrication (i.e., secondary molding is performed) by the female mold, and then continuous finishing is performed by the concave forming surface, so that an excessive force is prevented from being applied concentrically to the curved surface of the mold. Further, two or three films of vinyl each having a thickness of 0.02 mm are disposed between the magnesium plate and the female guide surface (that is, the concave guide surface and the concave forming surface) and therefore, no shrinkage or contraction is produced between the die (or molds) and the magnesium plate and a smooth folding fabrication can be achieved without any trouble.
Further, the forming method of the present invention comprises the steps of: placing a shrinkage restriction plate, which has an opening for receiving the male mold inserted therein, between the male mold and the female mold; placing at a lower portion of the shrinkage restriction plate a cantilever plate that is vertically movable in the female mold so that an upper surface of the cantilever plate is placed on the same plane as the female mold; providing a coupling rod so that the cantilever plate is disposed on an upper end of the coupling rod; providing a supporting base for fixing a lower end of the coupling rod; providing a resilient support member for supporting the supporting base; and inserting the male mold into the female mold, with a circumferential end portion of the magnesium plate being held by the shrinkage restriction plate. The shrinkage restriction plate can be fitted to either the male mold or the female mold.
Also, in the forming method of the present invention, the shrinkage restriction plate can be vertically movably mounted on the base plate for the male mold, and a spring element can be provided to downwardly bias the shrinkage restriction plate. If necessary, the female mold can be fitted at its upper surface by a fixing member.
Further, the forming method of the present invention utilizes a male mold or die that includes a generally rectangular surface having four corner portions, and a female mold or die having a correspondingly shaped opening. Each corner portion has a first side and a second side interconnected by an obliquely extending end surface. A first arc located within an angle formed by the first side and the end surface is tangent to the first side and the end surface at first and second locations, respectively. And, a second arc located within an angle formed by the second side and the end surface is tangent to the second side and the end surface at third and fourth locations, respectively. The first second, third and fourth locations along with a portion of the end surface located between the second and fourth locations are interconnected by a continuous curve.
In the method described above, several contact portions, that is five concentrations of force are provided at the corners or the curved surface portions to diffuse or disperse the concentration of the force to five cross points, so that the concentrated force can be dispersed. Therefore, the concentration of the force by the press working is dispersed to at least five locations so that generation of shrinkage or cracks on the curved portions can be prevented effectively.